http://www.pcoem.org/dam_failure.htm
"The dam is approximately 14,400 feet long and is generally between 45 and 60 feet high, reaching 90 feet at the old riverbed. The earth embankment has an impervious core and its section has slopes of 1 vertical to 2.5 horizontal. The crest of the dam is at elevation 145.0 ft. National Geodetic Vertical Data (NGVD), and is 24 feet wide. It accommodates a single-lane-paved road, which provides access to TRA personnel, law enforcement and emergency vehicles.
The dam is served by a concrete gravity spillway with an ogee crest at elevation 99.0 ft. NGVD. Releases are controlled by twelve 40-foot long by 32-foot high trainer gates. Concrete upstream and downstream aprons direct the water through the spillway and back to the original river channel.
The outlet works consist of a vertical inlet tower with five gates, a 550-foot long by 10-foot diameter conduit, a 170-foot long stilling basin, and a concrete broad-crested weir. A short channel downstream of the stilling basin directs flows back to the river.
Normal (conservation) pool level for the reservoir is 131.0 ft. NGVD, which encompasses a reservoir area of about 82,600 acres and storage of 1,750,000 acre-feet. Maximum pool design surcharge elevation is 134.0 ft. NGVD, which corresponds to a reservoir surface area of 88,900 acres and 2,045,000 acre-feet of storage. The drainage area above the reservoir is approximately 16,583 square miles and average flows are 7,440 CFS.
Livingston Dam is classified as a large, high-hazard structure with a recommended spillway design flood equal to the probable maximum flood (TDWR, 1978). According to the TDWR, the project will pass only 90% of the PMF before overtopping the dam."
I replied to this one rather than your latest because I think the answer toyour question is here.
" The crest of the dam is at elevation 145.0 ft."
"The dam is served by a concrete gravity spillway with an ogee crest at elevation 99.0 ft."
I don't think they can lower the level less than 46 feet below the crest without pumps. I know they can't using the spillway alone.
However:
"The outlet works consist of a vertical inlet tower with five gates, a 550-foot long by 10-foot diameter
conduit, a 170-foot long stilling basin, and a concrete broad-crested weir."
This may be a drain, and it may be a hydroelectric setup. Either way, the height of the lowest gate on the inlet tower will be the lowest level that they can drain the lake unless other draining systems not mentioned exist or else they use pumps.
(The rest of that part means the tower connects to 550 foot by 10 foot penstock, or pipe, which feeds into a sort of dammed up lake below the main dam so that racing water from the upper water level doesn't destroy riverbanks downstream, and the "dam" below the stilling pond is really a weir, which means a dam just below the surface to slow things down.)
BTW, that's a good sized dam. It normally holds 1.75 million acre feet of water, and can hold up to 2 million in a pinch. By comparison, Boulder/Hoover dam has a capacity around 27 million acrefeet. (1 foot of water an acre in size).
If this one failed at normal capacity, it would flood around 2200 square miles, or an area 50 miles by 50 miles, to a depth of one foot.